TSA and the Amtrak Police have a strained relationship at times. But the key point is that, unlike with airlines, people riding trains or buses don't have to go through security inspections and long lines at the stations. They just get on and ride. There are sometimes small teams roaming the platform at large stations asking to see ID and tickets, and sometimes luggage, but for the most part they're polite about it and seem to appear randomly (and not all that often). And of course the conductors do check IDs to go with tickets, on board. So it's security of a different kind, not the large-scale theater experienced at airports.